1. Field of the Invention
The present invention relates to a thyristor, and more particularly to a gate turn-off thyristor (GTO) having a multi-emitter.
2. Description of the Related Art
A gate turn-off thyristor is a thyristor which can not only turn on but also turn off a main current flowing between main electrodes (anode and cathode) by a control signal applied to a gate electrode in a state where a forward voltage is being applied between the anode and the cathode. In order to effectively perform the turn-off, it is preferable to narrow the area of the cross section of one current path so that the influence of the control signal can be effectively given to all positions of the current path. Also, in order to avoid a failure of the GTO, it is preferable to provide a plurality of current paths each of which is of an elongated stripe form having a predetermined length. A large current can be obtained by increasing the number of the stripes.
To show the general techniques concerning the gate turn-off thyristor, there are cited T. Yatsuo et al; IEEE Trans. ED, Vol. ED-31, No. 12, December 1984, pp. 1681-86, and U.S. Pat. No. 4,626,888 to T. Nagano et al issued Dec. 2, 1986 (filed Nov. 10, 1983, priority claimed Nov. 10, 1982), which are incorporated herein by reference.
In a typical GTO, a multiplicity of cathode emitters each of which is formed in the shape of elongated stripe and wherein each is enclosed by a gate electrode are arranged in a GTO pellet and each cathode emitter acts as an elemental GTO. A surface of the cathode emitter is metalized to provide a cathode (or emitter) electrode. An electrode plate is brought on the surfaces of the cathode electrodes of the pellet into pressure contact therewith so that all the elemental GTO's operate in a parallel manner.
However, in a large-current GTO using a large-diameter pellet, it is difficult to favorably form all of the cathode emitter regions. In order to allow the manufacture of large-diameter GTO's at a relatively high yield, an elemental GTO having a bad cathode is checked and eliminated from the use leaving only good elemental GTO's. For example, see U.S. Pat. No. 4,341,011 issued to Okano et al on Jul. 27, 1982, which is hereby incorporated by reference.
On the other hand, a heat dissipation from the GTO is affected by bringing a heat sink member such as radiator fin into pressure contact with the electrode plate. In order to maintain the thermal resistance of the heat sink member low, it is required to apply a pressure not smaller than a predetermined value to a surface of the electrode plate having a fixed area. At the pellet surface, this force equal to (area).times.(pressure) is mainly carried by a surface of the cathode (emitter) electrode. Therefore, it is preferable to make the occupation ratio of the cathode (emitter) electrode surface area to the pellet surface area as large as possible.
If the area of a surface portion of a cathode base exposed to the principal surface of the pellet and the area of a gate electrode provided on the surface of the cathode base increase, the area of a cathode emitter region and the area of the cathode electrode are correspondingly decreased so that the electrode material is liable to be crushed when a pressure is applied. Moreover, if the number of elemental GTO's eliminated as the result of a test is large, the force applied to the remaining elemental GTO's is necessarily further increased. Accordingly, the electrode material is further liable to be pressed thin and crushed. The crushed cathode electrode may extend to and electrically become in contact with the gate electrode or the surface portion of the cathode base region, thereby causing a short circuit.
In many cases, a level difference is provided on the surface of a pellet so that a gate electrode is provided at the lower surface level. This provision is effective for prevention of a short-circuit trouble. However, the division of a cathode emitter into many elemental regions results in the expansion of the length of a margin providing the difference level. A strain is liable to generate at the marginal portion, thereby causing mechanical damage such as breakage. Therefore, the expansion of the length of the level-difference margin is liable to lower the manufacture yield.
If no level difference is provided, on the other hand, a cathode emitter electrode, a gate electrode and a cathode base region assume a co-planar arrangement. In that case, if the cathode and gate electrodes contact with each other due to the crush, etc. of the cathode electrode when the pressure contact is made, there results in a short-circuit trouble. Even in the case where no short circuiting has resulted between the electrodes, short-circuit trouble is encountered if a pin-hole is present in a passivation film on the cathode base and the crushed cathode electrode extends into the pin-hole.
A buried gate structure has been proposed for static induction (SI) thyristors, gate turn-off thyristors, etc. (see, for example, U.S. Pat. No. 4,086,611, U.S. Pat. No. 4,171,995, U.S. Pat. No. 4,198,645 and JP-A-57-10971, which are hereby incorporated by reference). A p.sup.+ buried gate region of high impurity concentration is formed in a p.sup.- cathode base region of low impurity concentration, thereby permitting decrease of a distance between a current path and a region to be controlled.